ELECTRIC ARC HEATER

An electric arc heater (plasmatron) is a low-temperature plasma generator in which an arc discharge is used as a heat release element. An electric arc is a self-maintained gas discharge characterized by:

low cathode voltage drop (less than 20 V);

high current density (102 A/cm2 in the arc column and 103–107 A/cm2 in near-electrode zones);

temperature range of 5–50 kK and ionization degree of 1–100%.

In a plasmatron, such gases as air, inert gases, water vapor, natural gas can be used as a working fluid. Heating of the gas is accomplished by conductive, radiative and convective heat exchange between the arc and a gas. The electric arc is practically the only method of stationary heating of any gas to a temperature over 4 kK.

Depending on the purpose, various types of plasmatrons have been developed. These include linear (Figure 1a, b), coaxial; combined, multiarc and other types with both direct current (DC) and alternating current (AC). Plasmatron powers range from hundreds of W to tens of MW; arc currents, from several A to several kA; mass flow rates, from fractions of g/s to several kg/s; pressures, from 0.1 to 20 MPa. Plasmatrons show high arc-burning stability, are relatively small and the process of gas heating can be easily and automatically controlled. This is because an electric arc is a low inertia Ohmic heater.

Figure 1. Linear (a) and co-axial (b) plasmatrons.

The main characteristics and parameters of a plasmatron are as follows: current-voltage arc characteristic (CVC); thermal efficiency; specific erosion of electrodes, which determines a plasmatron’s lifespan; and heat flux. One of the main physical processes in a plasmatron is the by-passing phenomenon, i.e., unwanted electrical arc-wall and arc-arc (in a loop) contacts (breakdowns). Large-scale by-passing between an arc, placed along a channel axes, and a wall determines arc mean length and the maximum achievable electric power in the plasmatron of the type depicted in Figure 1a, sometimes called plasmatrons with self-aligning arc length. Large-scale by-passing also determines arc CVC, frequency of voltage pulsations ( Hz), etc. Small-scale by-passing in near-electrode zones affects the erosion rate and the dynamics of arc movement on electrode surfaces.

The main elements of a plasmatron are an anode, a cathode, a gas injection chamber and an arc channel. As a rule, an anode works in a form of water-cooled, hollow metal cylinder, which is part of the arc chamber. An arc moves into an inner surface of the chamber due to electrodynamic and aerodynamic forces (Figure 1a, b). Specific erosion of a copper cathode in an air medium for an arc current range of between 100–1,000 A is equal to 10−10–10−9 kg/s; in an argon, it is less by 2–3 orders of magnitude.

Two cathode types are mainly used, namely, the bar and the tube. Bar cathodes fabricated from refractory materials with high emission (W, C, etc.) are successfully used in the case of an inert gas (argon, helium, nitrogen, etc.) for an arc current range of 5–10 kA. A tungsten bar cathode, flush-pressed into a copper, water-cooled casing, effectively operates in an inert media at kA, p = 0.1 MPa and shows a minimal specific erosion of 10−13 kg/s. This is due to atom recirculation, that is, the return of cathode vaporized atoms to the cathode surface in the form of ions. This phenomenon reveals the wide opportunities for low-erosion cathode development.

In nitrogen or oxidizing media with current values ≤200 A, it is worthwhile to use cathodes made of hafnium or zirconium and flush-pressed into a casing. Tube cathodes of copper, bronze, cast iron, etc. operate well in oxidizing and other media at the current range of 100–1,000 A and at a specific erosion of kg/s. A method of decreasing erosion in a high-current tube cathode (e.g., tungsten cathode in argon medium) is the use of an auxiliary low-current arc, which provides preliminary generation of charged particles in a near-cathode zone. The specific erosion of such cathodes at the 3–8 kA current range doesn’t exceed (2 − 5) × 10−12 kg/s.

The electric field strength of an arc in a laminar gas flow is E ≤ 10 V/cm; in a turbulent flow, the strength is several times higher. That is why for the development of powerful linear plasmatrons it is worthwhile to use a turbulent arc. These plasmatrons require protection of the arc chamber walls from high heat fluxes. One of the effective means of doing this is by cold gas injection through the wall, which substantially decreases heat fluxes into the wall.

In the power balance of a plasmatron, the part determined by plasma radiation is appreciable. The spectrum of radiation determined by plasma temperature (103–105 K) corresponds mainly to the optical wavelength range. It consists of lines, a continuum and molecular bands.

At high arc currents and gas pressures, heat losses to the arc chamber walls—attributed to arc radiation—can be comparable with convective heat losses or can even be higher. One way to decrease radiative heat losses is to place a porous interelectrode insert and to inject plasma-forming gas through it. In a porous material, a regeneration of radiative and convective heat losses take place and these are returned to the main flow.

Another approach is based on arc turbulization by gas flow or magnetic field. In this case, an appreciable arc radiation decrease occurs.

In experimental plasmatron development and in associated theoretical investigations generalization of experimental data based on similarity criteria has a special importance. These criteria include both those well known from classical gas dynamics (Mach, Reynolds, Prandtl numbers, etc.) and also criteria peculiar to electric arc plasmatrons. Among the latter, the most significant criteria are the electric field strength criteria σED2/I or Su = σVD/I; the power criterion which shows a relation between Joule heat release and flow heat power, S1 = I2/(σρvH D3); the magnetic interaction criterion SB = IB/(ρv2D), which is important in the investigation of plasmatrons with magnetic control of an arc; and the criterion Sr = 4πεD2/(μH) shows the input of radiation power to the process of heat transfer in a plasma. Based on similarity criteria, the use of semiempirical methods for calculating plasmatron electrical and heat characteristics has been developed.

Plasmatrons are used for direct reduction of metals from ores, for metal cutting, for workpiece surface strengthening, for overlaying of corrosion-resistant, refractory and another coatings, for a synthesis of new materials, for the destruction and utilization of toxic wastes, as test facilites for aerospace purposes, etc.